The present invention relates to amplifiers, and more particularly to an amplifier circuit wherein the input signal is added to the power supply voltages which are applied to the amplifier so as to thereby improve the amplifier's rejection of common mode signals.
Pre-amplifiers are circuits for amplifying the signals provided by low level signal sources to a higher level suitable for further processing. In addition to providing gain, a pre-amplifier should exhibit a low output impedance and an input impedance selected in accordance with the type of signal source to which it is connected. The pre-amplifier should also provide little or no degradation of the signal-to-noise ratio of the signal being amplified.
One type of low level signal source with which pre-amplifiers are commonly employed is the phono cartridge used to convert the undulations in the grooves in phonographic records into electrical signals. A phono pre-amplifier should preferably have high input impedance so as to minimize the signal loading on the phonographic cartridge. It is desirable to use a differential amplifier for the input stage of a phono pre-amplifier in view of the high gain, low noise and good common mode rejection of differential amplifiers. There are, however, certain performance trade-offs in the selection of a particular differential amplifier configuration for this purpose.
Differential amplifiers have two inputs and ideally provide an output signal V.sub.o equal to a gain factor G.sub.d (usually referred to as the differential mode gain factor) times the difference between the signals applied to the two inputs of the amplifier. Unfortunately, practical differential amplifiers also have a small but nonzero common mode gain factor G.sub.c whereby the output also includes a component equal to the gain factor G.sub.c times the sum of the signals applied to the two inputs. The common mode component in the output signal is a distortion term and is undesirable.
In order to reduce the effect of the common mode gain of the amplifier, the amplifier should preferably be configured so that the common mode signal, i.e., the sum of the two input signals, is minimized. Unfortunately, the amplifier configuration which produces the lowest common mode signal also provides the lowest input impedance. Another amplifier configuration provides substantially higher input impedance, but has a higher common mode signal whereby a greater amount of common mode distortion exists in the output signal provided by the amplifier. A trade-off therefore exists between input impedance and common mode distortion in such differential amplifiers.